The goal of the Section on Drosophila Gene Regulation is to understand the regulation of homeotic gene function in Drosophila. The homeotic genes specify segmental identities in Drosophila at both the embryonic and adult stages. They encode homeodomain-containing transcription factors that control cell fates by regulating the transcription of downstream target genes. The homeotic genes are expressed in precise spatial patterns that are crucial for the proper determination of segmental identities. Both loss of expression and ectopic expression in the wrong tissues lead to changes in segmental identities. These changes in identity provide a powerful assay to identify the trans-acting factors that regulate the homeotic genes and the cis-acting sequences through which they act. Both the homeotic genes and the trans-acting factors that regulate them are conserved between Drosophila and man. In addition to many conserved developmental genes, at least half of the disease and cancer-causing genes in man are conserved in Drosophila, making Drosophila a very important model system for the study of human development and disease. Cis-acting transcriptional regulatory elements from the homeotic genes have been previously-identified by assays in transgenes in Drosophila. These assays have identified both tissue-specific enhancer elements, as well as cis-regulatory elements that are required for the maintenance of activation or repression throughout development. While these transgene assays have been important in defining the structure of the cis-regulatory elements and identifying trans-acting factors that bind them, their functions within the contexts of the endogenous genes is still not well understood. We have used a large number of existing chromosomal rearrangements in the Sex combs reduced homeotic gene to investigate the functions of the cis-acting elements within the endogenous gene. Characterization of the chromosomal rearrangements revealed that two genetic elements about 70 kb apart in the Sex combs reduced gene must be in cis to maintain proper repression. When not physically linked to each other, these elements interact with elements on the homologous chromosome and cause derepression of its wild-type Sex combs reduced gene. To validate our model, we have characterized a transposable element insertional mutation that was isolated 50 years ago and has very unusual genetic properties. The transposable element is inserted about 150 kb upstream of the Sex comb reduced promoter and we believe that the unusual genetic properties of this insertion derive from its ability to mimic the endogenous genetic elements required for transcriptional repression. We have identified the transposable element as the Drosophila Springer retrotransposon. We have used an unlinked genetic suppressor of Springer to show that the unusual genetic properties are actually due to the Springer insertion. Using a transgene assay, we have identified a 400 base pair region of the Springer retrotransposon that functions as a homeotic repression element. Using the same transgene assays, we are now trying to identify the cis-regulatory elements within the endogenous Sex combs reduced gene. Maintenance of repression from the homeotic repression elements requires the proteins encoded by the Polycomb group genes. We have identified a number of homeotic repressors, including the Su(z)12 and Mi-2 genes. In order to identify new Polycomb group repressors, we are screening for new mutations that either interact genetically with Polycomb mutations or mimic the homeotic phenotypes of Polycomb group mutations. We have generated almost six hundred new lethal mutants that die very late in development (after the formation of the adult cuticle during pupation). Among the new mutants with homeotic phenotypes are three alleles of previously-identified Polycomb group genes. Genetic studies have also identified the trithorax group of genes that are required for expression or function of the homeotic genes. Reduced function of the trithorax group genes mimics loss of function of the homeotic geness. Many of the trithorax group genes have been shown to be required for the maintenance of transcription of the homeotic genes during development. Many trithorax group proteins are subunits of chromatin-remodeling or transcriptional coactivator complexes. The section has identified at least two dozen trithorax group genes, most of which were not previously identified. We have identified several trithorax group genes that encode subunits of chromatin-remodeling complexes. The brahma, moira, and osa genes encode subunits of the Brahma chromatin-remodeling complex, which is conserved from yeast (the SWI/SNF and RSC complexes) to man (the BRG1 and HBRM complexes). To further understand the function of the Brahma complex, we have been characterizing mutations that interact with mutations in the Brahma complex. We have recently identified two other genes (taranis and tonalli) that show genetic interactions with Brahma complex mutations. We have also isolated mutations that show very strong genetic interactions with brahma, osa, taranis, and tonalli mutations. We mapped three of these mutations to the rhinoceros gene. Rhinoceros is required to maintain identify of the aristae. Loss of rhinoceros function causes cells to differentiate tarsal segments (distal leg structures) instead of aristae (distal antennal structures). The human homolog of rhinoceros is a transcriptional co-activator that physically interacts with the von Hippel-Lindau tumor suppressor protein.